Ultra high vacuum (UHV) systems require seals between joints that will prevent leakage down to a vacuum chamber pressure of substantially less than 10.sup.-8 torr. For low to medium vacuum (greater than 10.sup.-8 torr pressure), seals may be satisfactorily fabricated of elastomer materials. However these elastomer seals have been unsatisfactory in the past for higher vacuum systems.
UHV systems must generally be baked after each venting to air, typically at a temperature of 200.degree. C. for a period lasting 24 hours or more,to drive out gases from the walls of the vacuum chamber. Most elastomer seal materials undergo thermal setting under these bakeout conditions, and they have a permeation rate that equals or even exceeds the outgassing rate from the walls of the vacuum chamber. To pump these systems down to high vacuum requires an excessive number of vacuum pumps. Attempts have been made to reduce the permeation through the elastomer by manufacturing the elastomer in a different manner. Attempts have also been made to improve the seal by means of using grease. This technique has the unsatisfactory consequence that the vacuum system becomes contaminated by the grease material during bakeout. The prior art (FIG. 2) groove 43 design has proven to be unsatisfactory for using elastomer seals 17 from air 71 to UHV 20.
For this reason, UHV systems in the past have generally utilized metal seal technology, in which the seal material is a metal gasket such as copper. This technology provides ultra high vacuum-seals down to 10.sup.-12 torr pressure. However, metal seals are more expensive and inconvenient from the standpoint of the user. The seal must be replaced after each venting. The design of the metal-sealed joint is also more complex.
The effectiveness of an elastomer seal is primarily a function of two phenomena. First there is the phenomenon of permeation through the bulk elastomer material caused by microchannels or holes inherent in the material due to its chemistry and manufacturing processes. Second is the phenomenon of gas leakage around the seal at the elastomer/metal interface. The roughness of the metal surface in contact with the elastomer seal affects the amount of gas leakage. Typically, a surface finish of 32 microinches is regarded as adequate. The surface comprises a multitude of grooves with peaks and valleys, and the elastomer seals against the peaks but does not fill the valleys, even under high sealing pressure. These valleys provide channels through which gas may leak past the seal. Improving the surface finish by electropolishing alleviates, but does not eliminate, this leakage mechanism.
In addition, the metal surface is often produced by lathe machining, which generates a spiral groove pattern. This groove is a long continuous open path under the seal that allows a further gas leakage flow when used with elastomer seals.
The elastomer surface is itself rough, allowing for the leakage of gases along the surface of the elastomer.